Autism and Thimerosal: Is there really a correlation?

Autism and Thimerosal: Is there really a correlation? An article by Jonathan Mitchell

Autism epidemic autism soars! Headlines like these are common. A 1999 California report showed that between the years 1987 and 1999 the numbers of autistic persons who became clients of the State Department of Development services increased 273% in that period. From 1999 to 2003 the population of autistic persons doubled from about 10,000 to more than 20,000. In the 1970s the prevalence of autism was thought to be about one in 2500 persons. Recent studies have shown that the rate may be as high as one in 166. As a person with high functioning autism, I have followed these media reports with great interest.

There has been considerable debate over what has been causing the numbers to rise. One explanation is that autism is really just another name for mercury poisoning. More specifically many people believe that Thimerosal, a preservative in vaccinations that contains mercury, is the responsible culprit.

Several studies refuting the autism thimerosal connection have been published but the Anti-thimerosal crowd has always managed to poke holes in these.

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The controversy has resulted in numerous events. It has generated a commercially successful book, Evidence of Harm, by David Kirby. The movie rights for this book have been purchased. Also the matter of whether or not autistic children’s condition is the result of too many vaccines is being litigated in the court in class action suits involving thousands of people. There have been lobbies to ban Thimerosal from vaccinations at the federal and state level and several states, including California, have passed laws doing just that.

A treatment called chelation which takes heavy metals such as mercury out of the system using medications or creams has become an increasingly popular intervention for autism. An organization called Generation Rescue has even claimed that all autism is really mercury poisoning and that chelation is the cure.

The mercury activists have claimed the 2003 update to the 1999 California report on autism shows that there were great increases in admissions to the state’s department of developmental services among children who were born in the same years that more vaccines were added to the infant schedule. They believe this proves that the timing of two events is related. However, the old statistical saw of correlation not proving causation must be remembered. Still, if there is a true correlation between the two events, it can be justification for further research into the question to test the hypothesis even further.

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The year 1988 is a watershed in that prior to that time there was only one vaccine that contained thimerosal, the DTP vaccine. In his book, Evidence of Harm, author David Kirby claimed that starting in 1988 children received four shots of the Hib vaccine, the first administered at age two months. This is a factual inaccuracy. In 1988 the Hib vaccine was added to the schedule but there was only one shot and was not given to children under the age of 18 months. Therefore there were no infant vaccinations that contained thimerosal except DTP until 1991. It was not until that year that drastic changes in the vaccination schedule took place adding the four Hib shots which Kirby discusses as well as numerous others. Even in 1991 some of the Hib shots contained thimerosal but not all.

Is there really a correlation between increased exposure to Thimerosal in vaccines and increases in the prevalence of autism? Perhaps a less superficial look at the data is in order. The endpoints that are usually given by the media, by anti-mercury crusading groups and epidemiologists who are studying the problem are the birth years 1988 to 1997. However, a cursory examination of the 2003 update to the California report shows that the prevalence of autism more than doubled from 4/10,000 in the birth year 1970 to 9/10,000 in birth year 1986. The 1990 birth year’s prevalence was triple 1970's at 12/10000. It would appear that autism was well on the rise before there were increased exposures to thimerosal.

One possible explanation is that rates of DTP vaccination greatly increased during this time period. After all, there were laws passed during this time mandating vaccination for school attendance. Data from national surveys taken by the CDC contradict this notion. They show more than 76% coverage of children ages one through four in 1970 and about 74% in 1974. There is a downward trend in vaccination uptake for the next 11 years going to 65% in 1985.

The DPT is the only thimerosal vaccine that has been used in the United Kingdom which has shown autism prevalence rates comparable to the U.S. The UK DPT schedule was accelerated in 1990 so thimerosal exposure occurred at earlier ages. A UK infant receives the same cumulative mercury exposure at four months as one in America. After that age the exposure to thimerosal is less for the British babies than their American counterparts.

Funding for autism research to find cause and treatments is limited. The money spent on researching thimerosal could be better spent. Also, parents of autistic children are often vulnerable people looking for whatever quick fix is out there for their children; it seems wrong to raise their hopes.

Starting in 1999 thimerosal was phased out of infant vaccinations. The shelf-life of the last thimerosal containing vaccine expired at the beginning of 2003. Perhaps it’s time to let thimerosal rest in peace.